Alternatives to Insecticides in the Vegetable Garden


Plant Science Day. August 5, 1998.

By Kimberly A. Stoner
Department of Entomology; The Connecticut Agricultural Experiment Station
123 Huntington St.; P.O. Box 1106; New Haven, CT 06504.

Why do people use insecticides in the vegetable garden? To kill insects, obviously. But most insects in the garden are not pests. Some of these are beneficial insects, like honeybees, which benefit us because they pollinate flowers, or many insects that are predators of pests, such as ladybeetles or green lacewings. Other insects feed on organic material in the soil, or feed on nectar and pollen at flowers, or simply rest on the plants while passing through.

There are also insects that feed on vegetable plants, but they are never numerous enough or voracious enough to do significant damage, so these insects are not really pests. There was a book put out by the Experiment Station in the 1950’s called the Plant Pest Handbook. In revising this book, it was striking to me that for about one-third of the insects described feeding on vegetable plants, the authors in the 1950’s wrote " Control is not generally necessary." And for most of these insects, it is still true today: Control is not usually needed for many insects that just feed a little on the plant.

Some of the vegetable plants for which insect control is usually not needed are: beets, carrots, endive, garlic, horseradish, leeks, okra, parsley, parsnip, peas, rhubarb, scallions, shallots, and sweet potatoes. And you may find, through your own experience or the experience of other gardeners in your area, that there are other vegetable crops that are rarely bothered by pests where you live.

So, the first and most important alternative to insecticides is information. When you find an insect feeding on your plants, find out what it is, and whether it’s likely to do any important damage to the plant. If not, then there is no need to do anything about it.

Once you know what insect you have and what kind of damage it does, the next step is to decide on your level of tolerance for that damage. Commercial vegetable growers often don’t have the option of tolerating damage. When they bring their vegetables to market, the consumer usually expects them to look absolutely perfect. But, as a gardener, you are feeding yourself and your family (and maybe your friends and neighbors), and the standard of perfection is up to you. If it doesn’t bother you to have a few tiny little holes in a collard leaf, then you probably don’t need to control flea beetles on your collards in the fall.

However, there are insect pests that do serious damage to vegetable plants, and there are ways to manage these insects and reduce or eliminate the damage they do.

One simple method is to put a barrier between the insect and the plant. Spun-bonded row cover can keep out insects that fly in to feed or lay their eggs on garden plants. In order to make a good barrier, the edges of the material must be carefully buried, and the material supported if needed, so that it doesn’t rub against the plants or develop holes. It is also very important to combine the row cover with crop rotation, to avoid having pests from the same crop already present under the cover when the new crop is planted. Some of the pests that can be excluded with row covers are flea beetles, cabbage maggot flies, imported cabbageworm butterflies, cucumber beetles, and Colorado potato beetles. For some crops, like squash and melons, the row cover must be removed at flowering, so that the flowers can be pollinated and produce fruit. But that’s o.k. because the plants are usually most vulnerable to insect feeding when they are small. Cucumber beetles, for example, do their greatest damage to squash when the seedlings are first emerging from the ground. By the time the plants flower, and the covers must be removed, the plants are large enough to withstand a lot of insect feeding without losing yield. Dr. Martin Gent, of the Department of Forestry and Horticulture, has done experiments with row covers and their benefits in growing vegetables.

Another simple physical method is to remove insects by hand. Not all insect pests are easy to see and catch, but many that feed up on the leaves and are fairly large and slow-moving can be hand-picked in a small garden. Mexican bean beetles, Colorado potato beetles, and some caterpillars, such as the imported cabbageworm and tomato hornworm, are a few examples.

There are also sprays which have a physical effect on the insects, instead of chemically poisoning them. Soap sprays, which are called "insecticidal soap," but are really just specially formulated soaps, and are not toxic to humans, are used against small, soft-bodied insects, such as aphids. Ultrafine oils or horticultural oils are also used against small, soft-bodied insects. Soaps and oils work by blocking up the breathing holes and suffocating the insects or by damaging the protective layer of wax on their surface, causing the insect to dry out.

In addition to physical methods of control, there are biological methods. One biological method is to choose plant varieties that are resistant or tolerant to insects. One example is a variety of potato, available to the home gardener for the first time this past year, with the scintillating name NY L235-4. This potato has sticky hairs on its leaves that make it resistant to aphids, potato flea beetles, partially resistant to Colorado potato beetles, and especially resistant to a potato pest which has been abundant the last two years: the potato leafhopper. I compared NY L235-4 to a standard potato variety, Superior, several years ago (1993) when both potato leafhoppers and Colorado potato beetles were fairly abundant. There were significantly fewer leafhoppers per leaf and less "hopperburn" damage to the leaves on NYL235-4. I have seen this variety for sale for the first time from a small potato seed company in Maine in the last year.

Another biologically-based method of control is to use a spray based on a micro-organism that causes a disease in insects. One of these micro-organisms is the bacterium Bacillus thuringiensis. There are different strains of this bacterium that kill caterpillars or kill certain beetles. Thus, when shopping for a BT spray, you need to check the label to make sure that it will kill the species you have. A protein from this bacterium attaches itself to the gut of the host insect and causes it to stop feeding and die. There are different sprays available that are formulations of these proteins, with sprays specifically active against caterpillars and others active against Colorado potato beetles. When using Bacillus thuringiensis against either caterpillars or Colorado potato beetle, it is important to remember that it is easiest to kill these insects when they are small larvae. So you need to watch your garden carefully to get these insects soon after they hatch from their eggs.

Tiny nematodes that attack insects are also available. They are a different group from the nematodes that are sometimes pests on plants. These insect-attacking nematodes do not live very long out in bright light and dry air, so they are most effective against insects in or on the soil. There are several different species available, so you need to know which species are best for different purposes: some are better against insects on the soil surface, and others are better at finding insects in the soil. Again, you need to make sure you get the appropriate species for the pest species to be controlled.

Another form of biological control is to use predators and parasitoids that feed on your insect pests. A predator, like a ladybeetle or a spider, either hunts down or ambushes its prey, catches them, and eats them. A parasitoid hunts down its host insect and lays an egg in or on the host. The larva of the parasitoid then feeds on the host and eventually kills it. Then, the parasitoid turns into an adult, and the females hunt for another host in which to lay eggs.

The best way to use biological control is to encourage the predators and parasitic insects that are already in your garden. Gardeners have an advantage over most commercial growers because their gardens have a diversity of habitats, food resources, and target insects that can sustain a population of natural enemies.

One technique that I tested is using straw mulch on potatoes. The straw mulch reduces the abundance of Colorado potato beetle larvae. Further work on the effect of straw mulch on biological control agents showed that the straw increased the abundance of ground beetles. The ground beetles hide in the straw during the day and climb up the plants to feed on Colorado potato beetle larvae at night. On the other hand, mulching squash with either straw or plastic mulch tends to increase the population of squash bugs, again, by providing a place to hide. So mulching can provide habitat for a diversity of insects. You have to know for different crops whether the effect will be beneficial to you or to your pests.

Many insect predators and parasitoids occur naturally in the vegetable garden. It is important to learn to recognize them, so that you will know if they are numerous enough to control your insect pests for you.

There are companies that offer insect predators and parasitoids for sale. The one most commonly offered is a ladybeetle called Hippodamia convergens. I don’t recommend that gardeners buy this beetle, because many studies have shown that it does not stay around when it is released. It is ineffective except in a confined environment, such as a greenhouse. There are other insect natural enemies that are more effective, but they are usually too expensive to make sense for individual gardeners to buy for their own gardens.

I am working on project with a parasitoid of Mexican bean beetles, a tiny wasp, Pediobius foveolatus. One of the aspects I will look at is the cost of different sources and the feasibility of buying this parasitoid in large numbers to reduce the price. There is more information on this project at my plot..

Whether you are releasing purchased insect natural enemies or trying to encourage natural populations, it is crucial to avoid using insecticides with a toxic effect on a broad spectrum of insect species. So, as you learn to keep insect populations and damage to a tolerable level by using non-toxic physical and biological methods, you can help to keep insect natural enemies working to help you manage your insect pests.